To recover chemicals and generate steam, spent pulping liquor known as black liquor is usually burned in a recovery boiler. The organic compounds of the black liquor are recovered in the form of a smelt which can be regenerated to provide pulping liquor. Heat is thereby removed from the flue gas by heat transfer elements such as water filled tubes in which the water is converted into steam by indirect heat exchange. The heat content of the flue gas discharged from the furnace can be used to concentrate black liquor by evaporation by direct heat exchange wherein a product having a dry solids content of about 55% is coinverted into a product having a dry solids content of 65-70%. This method involves, however, disadvantages regarding the heat economy as well as environmental problems.
To avoid the drawbacks caused by the direct contact between the flue gas and the black liquor it has been suggested, as is described in the Canadian Pat. No. 917858, to concentrate black liquor to a dry solids content of 65-70% in a cascade evaporator by direct contact with a circulating steam flow, which is superheated by indirect contact with the flue gas from the recovery furnace. The recovery furnace is of conventional design and is provided with water filled tubes. The black liquor supplied to the cascade evaporator has been concentrated to a fairly high dry solids content in a conventional system.